A dynamic random access memory (DRAM) stores data by supply of electric charge to a capacitor. Thus, the off-state current of a transistor for controlling the supply of electric charge to the capacitor is preferably small. This is because off-state current can increase the data holding period, and the frequency of refresh operation can be reduced. Patent Document 1 describes a semiconductor device which can hold stored content for a long time by using a transistor including an oxide semiconductor film and having a low off-state current.